The Best Tank Line To Start With
Light Tank
Russian are the best, american light tank are most powerful, german tank are most manueverable (arguably)
Medium Tank
German MT are best at sniping role, with some of the most accurate cannon out of all nation. german also inherite the tradition of having gigantic lower glacies plate (easily penetrated part of front armor), which your enemy will alway attemp to exploit.
French MT are super flanker, their auto loader are good for people who like to do "hit n' run" style tactic (pop up and fire the whole clip, then pop back to reload). they have exceptional speed (not sure if faster than russian) allow them to flank with ease.
US MT are excel in dogfighting (brawl). their speed and firepower can be varied which make them versatile.
Russian MT are fast and manueverable, their gun are also good in both power and penetration. the main downside is that their tank can be prone to get ammo racked quite often (vulnerable ammo storage placement).
Heavy Tank
German HThave extremely thick armor and usually well sloped, their gun are the most accurate out of all nation (german tradition). they're usually slow and vulnerable agains artillery. and their gigantic lower glacies plate (another german tradition) make them vulnerable if that part ever expose.
French HT obtain the oscillating turret design and so is the aulo loading cannon, the gun on AMX 50B are almost identical to the american counterpart. French HT have very weak armor (tier wise) and probably lowest Hit point out of all three (if I recall).
US HT usually have very weak hull when compare to its counterpart, but have almost impenetratable turret (front) armor. to exploit this weakness the user should alway attemp to get in hull down position. so that only the thick turret may expose.
Russian HT have the best value for everything except accuracy and aim time (gun laying). so they're strongly recommended for close range dogfight to compensate for having weaker long range potential agains their counterpart. (that doesn't mean you should alway charge).
Tank Destroyer
German TD have the best value in armor thickness (until tier 8+) and are well sloped, which gave them high tendency to bounce off incoming shell. their gun are the most accurate (of course) and possess possibly the best penetration value. and just like other german vehicles, they also have the (bloody hell) gigantic lower glacies plate. (except ferdinand)
French TD french TD line just recently came so I can't say much about it,but at tier 10 they are just like the new WTF E100 with the autoloader that reload faster.
US TD have varied playing styles. The line starts out with a typical tank destroyer playing style with a great choice of guns, then leads to two tank destroyer lines; one with turrets and one mostly without turrets. Higher tier tank destroyers on the non-turreted line have extremely thick armor, but they are also extremely slow, thus making them vulnerable for arty fire. On the other hand, higher tier turreted tank destroyers tend to be very well armed but poorly armored. They are much more mobile compared to the non-turreted line. acting as support is advised.
Russian TD have much weaker armor than their counterpart and are not well sloped. their gun inherit russian tradition of having bad accuracy, but also the tradition of extreme firepower. late tier Russian TD gain access to the legendary BL-10 which posses the highest DPS and penetration value in the game.